Abstract: A fluid injection system and method for an internal combustion engine such as a spark-ignition or compression-ignition engine in which an injection nozzle injects a finely divided spray of fluid, such as water or a water solution, into the engine in response to a flow of atomizing air. The nozzle is connected to a fluid supply reservoir and to the outlet line of an air-injection pump that supplies pressurized air to the exhaust system of the engine. The air-injection pump provides the supply of atomizing air to the nozzle with the pressure of the air and therefore the fluid injection being responsive to both the engine speed and the exhaust gas pressure. The injected fluid advantageously functions as a cooling agent to suppress detonation and provide smoother engine operation and greater fuel efficiency.